Grammar fix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@55371 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/raw_ostream.h

@@ -220,7 +220,7 @@ public:
   ~raw_string_ostream();
   
   /// str - Flushes the stream contents to the target string and returns
-  ///  the strings reference.
+  ///  the string's reference.
   std::string& str() {
     flush();
     return OS;